1. Field of the Invention
The present invention relates to an optical information recording method, an optical information reproducing method, an optical information recording apparatus and an optical information reproducing apparatus in which servo information of a recording medium is read to record or reproduce information.
2. Description of Related Art
With recent increase in information amount, for an optical disc, high-capacity recording media and high-capacity recording methods that exceed the capacity of Blu-ray Disc have been vigorously studied. One of them utilizes holographic technology.
In a holographic recording method, interference fringes at intervals of a wavelength of light or less are recorded on a recording medium as a physical property change, and during reproduction, diffracted light from interference fringes of reproduction light is read. For example, Japanese Patent No. 3693990 (hereinafter referred to as “Patent Document 1”) discloses a technique in which a known holographic technology is applied to an optical disc recording technology. Two light sources of different wavelengths are used, one of which is used as a light source for recording information by hologram, and the other of which is used as a light source for servos.
The reason why the wavelength of the light source for servos is set to differ from that of the light source for hologram recording is not to form unnecessary interference fringes irrelevant to the recorded information within a recording layer. The wavelength of the light source for servos is thus required to be a wavelength that does not sensitize the hologram recording layer.
However, the provision of the two light sources of the different wavelengths within the optical information recording apparatus, not only results in cost disadvantage because of the necessity of two lasers, but also increases difficulty level in design because an optical system needs to correspond to the two wavelengths. Moreover, recording light reflects diffusely on a plane on which address information and/or servo information is recorded (servo information plane), which forms unnecessary interference fringes in the recording layer. In order to overcome the drawbacks, for example, Japanese Unexamined Patent Application Publication No. 2004-265472 (hereinafter referred to as “Patent Document 2”) proposes that a filter layer is provided between a servo pit plane and a recording layer. The filter layer transmits a wavelength of a light source for servos and reflects a wavelength of a light source for recording. However, the provision of this layer in a recording medium increases the manufacturing steps of recording medium, which results in cost-up.
On the other hand, a volume recording method different from the known hologram method has been proposed. For example, there has been made an attempt to apply a technique of recording reflective microholograms in layers to an optical disc as disclosed in the U.S. Pat. No. 6,020,985 (hereinafter referred to as “Patent Document 3”). This is a method in which two opposed light beams are collected in the same position within a recording material to form interference fringes only in a minute region in the vicinity of a focal point, and during data reproduction, one light beam is collected so that the data reproduction is performed depending on the presence or absence of diffracted light from the interference fringes.
As in the case of the known hologram recording, to record complicated interference fringes in a thick direction of the recording material, deformation of the interference fringes caused by heat expansion or contraction, for example, has significant influence on data reproduction. In this respect, the microhologram in which the interference fringes are recorded by bit-by-bit only in the vicinity of the focal point has an advantage that an acceptable amount of deformation of the interference fringes is larger than that of the known volume hologram recording.
On the other hand, regarding the positioning accuracy of the light beam, the recorded interference fringes themselves are large in the known volume hologram recording, so that the positioning accuracy of a diffraction limit level of light is not required. Accordingly, there is actually no problem in using a laser of a longer wavelength for positioning than the wavelength of the light source for recording.
However, unlike the known volume hologram, in the case of the microhologram that performs by bit-by-bit recording in a size similar to the diffraction limit of light, the positioning control of a light beam needs to be performed with a proper accuracy. That is, it is substantially difficult to perform the servo operation with the laser of the longer wavelength than the wavelength of the light source for recording.
Besides the example of the above-described microhologram, for example, there has been proposed a method of forming a minute refractive index changing region of a size similar to the diffraction limit within a recording material to record information as disclosed in Japanese Unexamined Patent Application Publication No. 2005-37658 (hereinafter referred to as “Patent Document 4”). In the positioning control of the light beam in the method, an accuracy similar to the diffraction limit of a laser for recording is also required as in the microhologram.